1. Field of the Invention
The present invention relates to a method and system for coding low bit rate speech for a communications system. More particularly the present invention relates to a method and apparatus for using a voicing measure to perform improved quantization at an encoder and additionally to enable the regeneration of a speech signal at the correct degree of periodicity at a decoder.
2. Background of the Invention
Currently, various speech encoding techniques are used to process speech. These techniques do not adquately address the need for a speech encoding technique that improves the modeling and quantization of a speech signal, specifically, the spectral characteristics of a speech prediction residual signal which includes a prototype waveform (PW) gain vector, a PW magnitude vector, and a PW phase information.
In particular, prior art techniques are representative but not limited to the following see, e.g., L. R. Rabiner and R. W. Schafer, “Digital Processing of Speech Signals” Prentice-Hall 1978 (hereinafter known as reference 1), W. B. Klejin and J. Haagen, “Waveform Interpolation for Coding and Synthesis”, in Speech Coding and Synthesis, Edited by W. B. Klejin, K. K. Paliwal, Elsevier, 1995 (heeinafter known as reference 2); F. Iatakura, “Line Spectral Representation of Linear Predictive Coefficients of Speech Signals”, Journal of Acoustical Society of America, vol 4. 57, no. 1, 1975 (hereinafter known as reference 3); P. Kabal and R. P. Ramachandran, “The Computation of Line Spectral Frequencies Using Chebyshev Polybimials”, IEEE Trans. On ASSP, vol. 34, no. 6, pp. 1419–1426, December 1986 (hereinafter known as reference 4); W. B. Klejin, “Encoding Speech Using Prototype Waveforms” IEEE Transactions on Speech and Audio Processing, Vol. 1, No. 4, 386–399, 1993 (hereinafter known as reference 5); and W. B. Kleijn, Y. Shoman, D. Sen and R. Hagen, “A Low Complexity Waveform Interpolation Coder”, IEEE International Conference on Acoustics, Speech and Signal Processing, 1996 (hereinafter known as reference 6). All of the references 1 through 6 are herein incorporated in their entirety by reference.
The prototype waveforms are a sequence of complex Fourier transforms evaluated at pitch harmonic frequencies, for pitch period wide segments of the residual, at a series of points along the time axis. Thus, the PW sequence contains information about the spectral characteristics of the residual signal as well as the temporal evolution of these characteristics. A high quality of speech can be achieved at low coding rates by efficiently quantizing the important aspects of the PW sequence.
In PW based coders, the PW is separated into a shape component and a level component by computing the RMS (or gain) value of the PW and normalizing the PW to a unity RMS value. As the pitch frequency varies, the dimensions of the PW vectors also vary, typically in the range of 11–61. The quantization of the PW magnitude vector can be performed more accurately by utilizing the knowledge about the degree of the periodicity of the speech signal.
In prior art, the SEW and REW quantized magnitude components and SEW and REW estimated phase components are used in regenerating the speech signal. The use of estimated phase components result in speech that is rough and not very accurate due to the approximations used. References 1–6 do not adquately present a solution to this problem.
Thus, a need exists for a system and method that provides a means to characterize and encode the degree of periodicity of the input speech signal and at the decoder, use this information to regenerate the speech signal with the correct degree of periodicity.